The present invention relates to a motorcycle hydraulic brake system with a brake force regulator, in which in a first brake circuit a pedal master cylinder acts on a first brake caliper of a dual-circuit front-wheel brake as well as on a rear-wheel brake, in which in a second brake circuit a hand master cylinder is provided which acts on a second brake caliper of the front wheel brake, and in which the brake force regulator comprises a pressure reducing valve inserted in the first brake circuit and controllable by the pressure of the second brake circuit.
A motorcycle brake system of this type is described in German Pat. No. DE-OS 2,558,825. The brake force regulator used in this system resembles conventional brake force regulators for passenger cars. By actuating the pedal master cylinder, pressure is initially supplied to a front-wheel brake unreduced and to the rear-wheel brake reduced. If in addition the hand master cylinder is actuated, the pressure thereby generated counteracts the control force of the brake force regulator so that the latter reduces the pressure of the rear-wheel brake to a greater extent than if only the pedal master cylinder were actuated. In this arrangement the brake force regulator is so designed that the displacement travel of its stepped piston is sufficient to reduce the pressure in the rear-wheel brake by means of an increase in the pressure of a front-wheel brake.
The prior known motorcycle brake system has the functional disadvantage that its stepped piston may be displaced also when pressure is built up solely by the hand master cylinder, i.e., when only the hand brake, which acts on the front wheel, is applied. The displacement of the stepped piston causes pressurized fluid to be drawn from the rear-wheel brake. As a result, a vacuum will occur in the rear-wheel brake which may draw in air and impurities that can result in a failure of the brake.
It is a further disadvantage of the known arrangement that on actuation of the hand master cylinder the stepped pistion is required to be displaced against the pressure of the pedal master cylinder. This means that full reduction of the pressure of the rear-wheel brake can be achieved only if the operator's pedal force is reduced simultaneously with the actuation of the hand master cylinder, thereby increasing the volume of the pedal master cylinder. In particular in a panic situation, the operator cannot be expected to react so as to apply a metered measure of brake force.